Such antennas are disclosed in a number of patent publications of the present applicant, including GB2292638A, GB2309592A, GB2310543A, GB2338605A, GB2346014A GB2351850A and GB2367429A. Each of these antennas has at least one pair of diametrically opposed helical antenna elements which are plated on a substantially cylindrical electrically insulative core made of a material having a relative dielectric constant greater than 5. The material of the core occupies the major part of the volume defined by the core outer surface. Extending through the core from one end face to an opposite end face is an axial bore containing a coaxial feed structure that comprises an inner conductor surrounded by a shield conductor. At one end of the bore the feed structure conductors are connected to respective antenna elements which have associated connection portions adjacent the end of the bore. At the other end of the bore, the shield conductor is connected to a conductor which links the antenna elements and, in each of these examples, is in the form of a conductive sleeve encircling part of the core to form a balun. Each of the antenna elements terminates on a rim of the sleeve and each follows a respective helical path from its connection to the feed structure.
Some of the above prior patent publications disclose quadrifilar helical antennas intended primarily for receiving or transmitting circularly polarised electromagnetic waves. Each of these antennas has four helical tracks plated on the cylindrical surface of the core, or four groups of helical tracks, each group forming a composite antenna element and comprising two tracks separated by a narrow slit.
Whether the antenna has four helical antenna elements or two, the connection portions connecting the antenna elements to the feed structure conductors are radial tracks plated on a planar end surface of the core.
It is known to provide a quadrifilar helical antenna with an impedance matching network. This may be embodied as a small printed circuit or laminate board secured to the top end face of the core where it provides coupling between the feed structure and radial connection portions such as those disclosed in the above mentioned prior patent publications. An antenna having such a matching network is disclosed in our co-pending U.S. patent application Ser. No. 11/472,587. The disclosures of this application and each of the prior patent publications referred to above are specifically incorporated in this specification by reference.
It is an object of the invention to provide an improved dielectrically-loaded antenna.